Carbon monoxide is a tasteless, odorless, and colorless gas that interferes with oxygen delivery to your body's tissues. It's dangerous in any quantity, but even more so under pressure. Symptoms of carbon monoxide poisoning include headaches, nausea, blue lips and fingernails, or unconsciousness. Immediate medical attention is necessary no matter how severe the symptoms appear to be.

Two common causes of carbon monoxide contamination of your breathing air are exhaust fumes and poorly maintained air compressors. Reputable dive shops take extensive measures to prevent contamination of your air supply, and this is why you should only have your cylinders filled by qualified dive shops. You should also smell your air before diving. You cannot smell carbon monoxide itself, but oily or other chemical odors are often a sign of possible carbon monoxide contamination.